Polylactic acid or polylactide (a.k.a. PLA) is a biorenewable, biocompatible and biodegradable thermoplastic polyester derived from the cyclic ester lactide. Polylactide is well-known to be useful in a variety of applications in a wide variety of fields and technologies. Examples of useful poly-D-lactide (PDLA) and poly-L-lactide (PLLA) products include, but are not limited to, upholstery, microwavable trays, clothing materials and fabrics, engineering plastics, medical devices (e.g., sutures, stents) and drug delivery formulations. Polylactide biodegradability properties make it useful for disposable packaging materials and disposable items, such as garments and feminine hygiene products.
Polylactide can be derived from renewable resources, such as corn starch or sugarcane. Typically, bacterial fermentation can be used to produce lactic acid from corn starch or cane sugar. Because of the difficulties associated with achieving high molecular weight, the direct polymerization of lactic acid is not ideal for producing useful products. Lactic acid is, therefore, used to prepare oligomers which are then dimerized using a catalyst to prepare a cyclic lactide monomer. Ring-opening of lactide leads to higher molecular weight polylactide using stannous octoate catalyst or tin (II) chloride, for example.
There has been interest in the polymer field of preparing polylactide derivatives that exhibit enhanced properties as compared to unmodified polylactide, such as higher glass transition temperature, improved thermal stability, controlled crystallinity, reduced water absorption, increased toughness, and the like. Efforts to derivatize polylactide have often been associated with the objective of producing a polylactide that mimics the benefits associated with petroleum-based polymers while still possessing “biofriendliness.” Alkly-, alkenyl-, and aryl-substituted glycolides and glycolides with protected pendant carboxyl, hydroxyl, and amino groups have been prepared from corresponding alpha-hydroxyacids of variable origin and polymerized. Generally, polylactide-containing materials continue to be of significant interest in the field of consumer materials such as containers and packaging, due to their biorenewability, biocompatibility and biodegradability properties. Difficulty has been encountered, however, in balancing the biodegradability of materials with durability and toughness.
Poly-L-lactide (PLLA) can have a glass transition temperature (Tg) from about 50° C. to about 70° C. and a melting temperature between about 173° C. and about 178° C. One disadvantage with existing polylactide derivatives is that the resultant polylactide exhibits relatively low glass transition temperatures and is, therefore, not suitable as a containment or packaging material that experience elevated temperature environmental conditions and/or contents. Another disadvantage associated with polylactide-based materials is their long-term durability, brittleness, and cracking over time.
There exists a need in the polymer field for improved lactide monomer derivatives and polymerization processes that could utilize such monomers in the preparation of polylactide-derived materials. There further exists a need in the field of polymer products for thermoformable materials that are capable of withstanding elevated temperature conditions while still exhibiting biorenewability, biocompatibility and biodegradability. Still further, there is a need to develop materials that balance the biodegradability of the polylactide material with the toughness, durability and water transport control properties desired for its use for containment and packaging.